The Effects of Ampicillin on the Growth of Escherichia coli
Khadija Lawrence and Michelle Anthony
North Carolina State University, Department of Microbiology,
4510 Thomas Hall, Campus Box 7615 Raleigh, NC 27695
Abstract
Escherichia coli is a Gram-negative, rod-shaped bacterium that
normally inhabits the intestines of humans and animals. Not all
strains of E. coli are pathogenic, but ones that are contribute to
one of the leading causes of foodborne illnesses. (2) Given this
information, it would helpful to know if an antibiotic such as
Ampicillin has any effect on the growth of E. coli. Ampicillin is a
beta-lactam antibiotic that attacks Gram-positive and some Gramnegative bacteria. The amino group in Ampicillin allows it to
penetrate the outer membrane of Gram- negative bacteria. It will
then become an inhibitor of transpeptidase, which is needed for
bacterial cell wall formation,
and eventually leads to cell
lysis. (1)
Figure 1: Ampicillin
Structure (1)
Methods/Materials
Several preliminary
experiments were used in order to determine if E. coli was
sensitive to Ampicillin. Once this was successfully achieved
varying amounts of liquid ampicillin were used to see if the IC50
could be determined. Throughout experiments, the amount of
ampicillin added to samples ranged from 0-200 µL. Each sample
mixture contained H20, low salt concentration LB and the
inoculum, totaling 250 µL per sample. There were approximately
6-8 different samples used for each experiment. After making all
sample mixtures, 200 µL of solution was loaded into wells to be
measured in the Bioscreen C growth curve analyzer.
Results
Performing the experiment three times, a trend was discovered
between the lag and log phase. Through graphing, as seen in
Figure 2, the slope of the log phase was shown to be consistent
throughout the varying amounts of ampicillin. However, the lag
phase was most affected. As the amount of ampicillin was
increased, the distance between the lag phases increased also in
a correlating manner. At the higher concentrations, there
appeared to be no growth because the lag phase was delayed so
much. The graphical correlation of Ampicillin to the length of the
lag phase can clearly be seen in Figure 2.
Absorp'on 1.4 1.2 1 0.8 0.6 0.4 0.2 0 E. coli Growth vs Increasing Amounts of Ampicillin 0 3 5 9 12 16 2500 Time (Minutes) 2000 Increasing Ampicilin Concentration on E. coli Lag Phase 1500 1000 500 0 0 5 10 15 20 Ampicilin (MicroL) Figure 3: Displays increasing amounts of ampicillin (in
250microL mixtures) affects the length of the lag phase
Discussion
There were some difficulties with the experiment. When the
experiments were first started we had focused on using
serial dilutions of ampicillin to find the IC50. The titrations
did not show a great enough effect on the E. coli. Later, we
settled on using varying amounts of ampicillin to find a
correlation between lag and ampicillin. From varying the
amounts of ampicillin, we found the max and the slope of
the log phase stayed the same. The lag phase was most
affected for as the amount of ampicillin increased, the
amount of distances between the lag phases increased
also. After 16 µL, the lag phase cannot be measured as the
time it takes for it to begin growing was outside the time
frame of the experiment. The reason why the E. coli was
most affected in the lag phase and nowhere else had to do
with the cells trying to adapt to their environment. As their
environment became more filled with ampicillin and less
filled with water, the E. coli cells took more to adapt and
mature. This caused the delay within the lag phases.
Conclusion
The lag phase of E. coli is most affected by the varying
concentrations of ampicillin. In future experiments, we could test
E. coli in different environments, like one similar to humans or
farm animals, to see if the conclusion is the same. We also could
see if there was a way to have the lag phase affected at lower
concentrations by taking nutrients, like the Luria broth, away.
Acknowledgements
We would like to thank Dr. James Brown for his guidance of our
research and the NCSU Microbiology Department for supplies.
References
(1) Ampicillin/sulbactam. (2013, March 14). Retrieved December 3, 2013,
from Wikipedia: en.wikipedia.org/Ampicillin/sulbactam
Time Figure 2: Measured absorptions as ampicillin is
increased to display the effects on E. coli growth (2) Escherichia coli (E. coli). (2012, August 3). Retrieved December 3,
2013, from Centers for Disease Control and Prevention
www.cdc.gov/ecoli/general/